Helicopter rotor

ABSTRACT

Rotary-wing aircraft rotor comprising a rigid hub coupled to the root of each blade in which a trailing return brace comprises a stack of metal plates alternating with plates of visco-elastic material the ends of which being coupled via a ball joint respectively to the root of one blade and to a place of the hub such that the brace is always slightly inclined.

The invention relates to a rotary-wing aircraft rotor comprising a rigidhub to which the root of each blade is coupled by means of a laminatedspherical abutment and a resilient trailing return brace.

French Patent Specification No. 73.25319 published on Feb. 1, 1974describes a rotary-wing aircraft rotor having a completely rigid hubhaving a number of radial tubular arms equal to the number of blades;the root of each blade engages inside the corresponding tubular arm, towhich it is coupled inter alia by a spherical abutment, which likewiseis inside the radial arm; a hydraulic or pneumatic drag shock-absorberhas one end connected by a ball joint to the foot of the blade inquestion inside the corresponding tubular arm and the other endconnected by a ball joint to a place on the outside of the radial armassociated with an adjacent blade; to this end the shock-absorber rodmust extend through a wide aperture in the base of the wall of thetubular arm associated with the blade in question, the aperture beingfitted with sealing means. The hub has numerous disadvantages. Itsradial tubular arms are subjected to bending and tensile stresses, thelatter due to centrifugal force acting on the blades, and must haverelatively thick walls, which considerably increases the weight of therotor assembly. Manufacture is expensive owing to its complicatedstructure (complicated mould, numerous machining and inter aliafinishing operations). The aperture required in the wall of each tubulararm for the corresponding shock-absorber rod greatly reduces the fatiguestrength of the arm, when subjected to considerable dynamic stresses.When the rotor starts and stops, the blades are not brought to theirneutral position corresponding to a zero angle of drag, since theshock-absorbers do not exert any resilient return force on the blades.Likewise, the range of spring movement of each trailing blade is notlimited until the piston of the corresponding shock-absorber abuts theend of its cylinder. Consequently the known rotor blades can occupy awide variety of positions when the rotor starts or stops. This mayresult in a considerable unbalance during the aforementioned phases ofoperation, thus exciting the rotary-wing aircraft assembly at therotation frequency of the rotor. The result may be a resonancephenomenon peculiar to rotary-wing aircraft and known as "groundresonance", which may seriously damage the aircraft before take-off orafter landing.

U.S. Pat. No. 4,028,001 of Kenneth Watson, issued June 7, 1977 andentitled "Rotor For Rotary Wing Aircraft", likewise describes arotary-wing aircraft rotor comprising a completely rigid hub which, asbefore, has a number of radial arms equal to the number of blades. Eachradial arm has an internal cavity opening from both sides of the arm sothat a U-shaped component can be inserted into the cavity, the outerarms of the U being secured to a sleeve attached to the correspondingblade root; the U-shaped component is coupled to the corresponding armof the hub inter alia by a spherical abutment, likewise mounted in thearm cavity; elastomeric drag shock-absorbers each have one end coupledto the sleeve connected to one blade root and the other end secured by aball-joint link to the central part of the hub, thus securing the hub tothe rotor strut. Consequently the last-mentioned shock-absorbing deviceis complicated, heavy and expensive. Owing to its structure, the knownrotor hub must be considerably thicker than each blade in the directionof the rotor axis, so that the hub is inevitably heavy, plus the weightof the U-shaped components and sleeves secured thereto. Owing to itscomplex structure, the known hub is also very expensive to manufacture.

U.S. Pat. No. 3,923,419 of Rene Louis Mouille, issued Dec. 2, 1975 andentitled "Damped Elastic Tie Device Between Rotor Blade and Hub onRotary-Wing Aircraft", describes a resilient shock-absorbing connectingdevice between a rotary-wing aircraft rotor blade and the rotor hub. Oneembodiment of the device comprises a resilient component and ashock-absorbing component combined in the form of at least one set ofparallel metal plates between which there are layers of a viscoelasticmaterial having high rigidity and persistence of deformation. However,the last-mentioned connecting device is applied to a rotor having a hubprovided with arms to which the blade roots are coupled by purelymechanical trailing joints.

The rotary-wing aircraft rotor according to the invention is of thepreviously-described kind but does not have any of the aforementioneddisadvantages of the known rotors.

The rotor according to the invention is characterised in that trailingreturn braces each comprise a stack of metal plates alternating withplates of visco-elastic material having great persistence of deformationand form frequency adapters, one end of each brace being coupled via aball-joint to the root of one blade and the other end being coupled by aball-joint to a place on the hub such that the brace is always slightlyinclined to the corresponding blade and the centre of one ball joint isnear the beat axis of the blade, which extends through the centre of thecorresponding laminated spherical abutment.

In a first embodiment of the rotor according to the invention, theperipheral part of its hub is a flat ring having a convex polygonal orsubstantially circular periphery and formed in the direction of therotor axis with a number of apertures equal to the number of blades,each laminated spherical abutment is mounted between the outer edge ofone such aperture and the ends of the arms of a forked component securedto the corresponding blade root, and the second end of each trailingreturn brace is coupled via a ball joint to a place on the hub peripherybetween a laminated spherical abutment associated with the blade inquestion and the abutment associated with the immediately preceding orfollowing blade in the direction of rotation of the rotor.

The rotor hub according to the invention comprises a flat annularperipheral part to which the blade roots are coupled inter alia byforked members co-operating with the corresponding spherical abutmentsvia apertures in the peripheral part of the hub, the apertures extendingin the direction of the rotor axis. Consequently the flat annularperipheral part of the hub, in the direction of the rotor axis, can beconsiderably thinner than each blade. Furthermore, since the rotor hubaccording to the invention does not have thick-walled radial tubulararms, it can be much lighter than prior-art rotor hubs of the same kind.Owing to its simple structure, the rotor hub according to the inventioncan be cheaper to manufacture. Since each rotor blade according to theinvention is coupled by a resilient return brace of thepreviously-described kind, the plates of visco-elastic material subjectit to a considerable return force towards its neutral positioncorresponding to a zero angle of drag. Consequently, when the rotoraccording to the invention starts and stops, all its blades are in theirrespective neutral positions, thus reliably preventing an appreciableunbalance which can give rise to the dangerous phenomenon called "groundresonance".

In addition, the resilient return braces act as adapters of the naturaldrag frequencies, since, owing to their resilient properties, they canbe used to adjust the frequency of the first natural mode of dragvibration of each blade to a value sufficiently below the frequencycorresponding to the rated rotation speed to prevent any risk ofresonance when the rotor rotates at normal speed, but sufficiently highto eliminate the problems of ground resonance.

Finally, owing to the great persistence of deformation in theelastomeric material forming the visco-elastic shock-absorbingcomponents of the braces, the motion of the trailing blades is greatlyreduced, more particularly when the rotor starts or stops, when therotation speed passes through the value corresponding to the naturalfrequency of the first vibration mode of the trailing blades. Thiseliminates any risk of resonance phenomena.

Other known rotary-wing aircraft rotors have a substantially rigidcentral part of the hub formed in the direction of the rotor axis with anumber of apertures equal to the number of blades, a spherical abutmentalso being mounted between the outer edge of each aperture and the endsof the arms of a forked component secured to the corresponding bladeroot. However, the last-mentioned known rotor hubs comprise an outerstar-shaped part having a connecting arm for each blade, the arm beingflattened in the plane of the star and flexible in the directionperpendicular thereto. Consequently, a star-shaped hub of theaforementioned kind has a much greater diameter than the rotor hubaccording to the invention, which has a convex polygonal or circularperiphery. As a result, the known hub has a greater drag, other thingsbeing equal, than the rotor according to the invention, so that thelatter uses energy much more efficiently. At a given fuel consumption, ahelicopter equipped with a rotor according to the invention can reach aspeed greater by about 2% in the case of a light aircraft and about 5%in a heavier aircraft. Furthermore, owing to their great flexibility,the flexible arms of the known rotors can cause difficulty duringstarting and stopping of the rotor in a high wind. Since the rotor hubaccording to the invention has a considerably reduced diameter, theblade roots can be secured near the rotor axis, thus further reducingthe drag of the rotor according to the invention and facilitatingstreamlining thereof, for the same purpose. Owing to its reduceddiameter and the absence of arms, the rotor hub according to theinvention can be considerably lighter than a comparable star-shaped hub.The rotor hub according to the invention is also much simpler andtherefore cheaper to manufacture than a star-shaped hub. Finally, owingto the absence of a flexible arm, there is a considerable reduction inthe eccentricity of the beat of the blades and, other things beingequal, the rotor control power is reduced by about 25% compared with astar-shaped rotor, resulting in a considerable reduction in thevibratory excitation of the rotor according to the invention.

In a preferred embodiment of the rotor according to the invention, theforked component coupling each blade root to the corresponding sphericalabutment is a prolongation of the blade root, the end of the arms beingsecured, e.g. by two bolts, to the holder of the laminated sphericalabutment. The last-mentioned embodiment is particularly advantageous inthat each blade root can be brought as near as possible to the rotoraxis and the number of rotor components can be reduced.

In another embodiment of the rotor according to the invention, theforked component mainly comprises a radially disposed yoke, the end ofthe fork further from the hub being secured to the corresponding bladeroot e.g. by two shafts substantially perpendicular to the rotor planewhereas the other end of the yoke comprises two rigid flat elementssecured to one another, disposed on either side of the peripheral partof the hub without contact therewith and secured, e.g. by two bolts, tothe holder of the corresponding spherical abutment; for example, theyoke mainly comprises two rigid plates secured to one another andsubstantially parallel to one another and to the spherical part of theflat annular rotor.

The last-mentioned embodiment is particularly advantageous in that it isspecially suitable for constructing a rotor having foldable blades; tothis end, it is sufficient if one of the shafts securing the yoke to theblade root is movable so that the blade can be folded in the rotor planeby pivoting around the other shaft securing the yoke. Since the yoke,inter alia the two rigid parallel plates forming it, can still berelatively short in the radial direction, the last-mentioned embodimenthas practically the same advantages as the preferred embodiment.

Numerous kinds of rear rotors for helicopters are already known. Somehave mechanical joints provided with ball, roller or needle bearings; inothers, the flexibility of the blade-securing devices is used to allowthe blades to beat and move in step. A rear rotor of the last-mentionedkind is described in French Patent Specification No. 2 315 432, filed onJune 22, 1976. The rear rotor has four blades and inter alia comprisestwo flexible blade-shaped longitudinal members disposed perpendicular toone another, their central parts being held between two plates securedto the rotor shaft, the members forming the hub thereof. The rotor alsocomprises four sectional shells each surrounding half a longitudinalmember, to which they are connected to so as form the four blades.However, although the longitudinal members are made of high-strengthfibres coated with a synthetic thermosetting resin, their life islimited owing to the very high stresses to which they are subjected,mainly owing to the bending motion corresponding to the beat of theblades, and to torsion corresponding to the blade pitch control. Inaddition, the method in question of controlling the blade pitch requiresconsiderable force during manoeuvring, which necessitates the use ofservo-control systems, which are frequently duplicated for safety.Furthermore, damage to a single blade, inter alia to the correspondingpart of the longitudinal member, will mean that the entire member (i.e.two blades) will have to be changed. Finally, the fact that the bladesextend through the centre of the hub complicates the design of theconventional pitch control means using a central rod, since it is notadvisable to convey the rod through an orifice in the central part ofthe blades, where they are subjected to high stresses. In such cases, asstated in Patent Specification No. 2 315 422, numerous precautions haveto be taken in constructing the aforementioned part of the hub.

The rear helicopter rotor according to the invention does not have anyof the aforementioned disadvantages. Its blades are each coupled to thehub by a laminated spherical abutment having three degrees of libertyand are not subjected by the control forces to excessive stressescapable of reducing their life. There is a great reduction in the forcewhich has to be applied to each blade to vary its pitch, since it hasonly to overcome the internal resistance of the corresponding laminatedspherical abutment. When a rotor blade according to the invention isdamaged, it can be replaced without another blade having to be replacedat the same time. Finally, it is still possible to recess the centralpart of a rear rotor hub according to the invention to provide a freepassage for the pitch variation control device, which can thus be madesimple, compact and light and adapted to be streamlined at the same timeas the hub, in order to reduce the rotor drag. In spite of theseadvantages, the rear helicopter rotor according to the inventionrequires practically no maintenance, except for possible replacement ofthe laminated spherical abutments and trailing return braces after longuse.

Another embodiment of the rotor according to the invention comprises ahub having a different, simpler and mechanically more rugged structureand also more compact, thus reducing the aerodynamic drag. Thelast-mentioned rotor according to the invention is characterised in thatits hub has a central stem prolonging the rotor strut and bearing a topplate and a bottom plate, one rigid component of each laminatedspherical abutment is fitted between and directly secured to the edgesof the two plates to form a rigid cross-member, the root of each bladeis secured to the other rigid component of the corresponding sphericalabutment by a radially-disposed yoke which is recessed to provide a freepassage for the spherical abutment, and the first and the second end ofeach trailing return brace are coupled by ball joints, the first end tothe yoke associated with the corresponding blade and the second end to asuitable place on the central stem of the hub.

Since the top and bottom plates of the rotor hub according to theinvention are not recessed to provide a free passage for sphericalabutments, and since their respective edges are braced by the rigidcomponents of the spherical abutments, inserted between them, the hubplates have excellent mechanical resistance to the mainly radial tensileforces resulting from the centrifugal force on each blade, and also tothe static and dynamic bending moments of beat and drag, exerted by therotating blades on the means connecting them to the hub.

In the last-mentioned embodiment, the top and bottom hub plates arepreferably thin and the bottom surface of the bottom plate has e.g.radial reinforcing ribs. The last-mentioned advantageous feature enablesa reduction to be made in the weight and cost of the rotor hub accordingto the invention without reducing its mechanical strength.

By way of example, a number of embodiments of the rotor according to theinvention are described hereinafter and diagrammatically illustrated inthe accompanying drawings, in which:

FIG. 1 is a plan view of a blade root of a main helicopter rotoraccording to the invention, showing the part of the rotor hub to whichthe blade root is coupled;

FIGS. 2 and 3 are sectional views along lines II--II and III--III ofFIG. 1 respectively;

FIG. 4 is a plan view of another embodiment of a main helicopter rotorhaving four blades foldable in the rotor plane;

FIG. 5 is a section along line V--V of FIG. 4;

FIG. 6 is a partly cut-away front elevation of a rear helicopter rotoraccording to the invention;

FIG. 7 is a section along line VII--VII of FIG. 6;

FIG. 8 shows part of another embodiment, in section through an axialplane through the hub extending along the longitudinal axis of oneblade;

FIG. 9 is a partial view of the same embodiment in section along lineIX--IX of FIG. 8;

FIG. 10 is an exploded view of the recessed yoke which, in FIGS. 8 and9, connects the blade root to the corresponding spherical abutment;

FIG. 11 illustrates a variant of the hub shown in FIG. 8;

FIG. 12 is a section, corresponding to FIG. 9, of another embodiment ofthe rotor according to the invention, and

FIG. 13 is a perspective view of the cross-member forming part of therecessed yoke in the embodiment illustrated in FIG. 12.

FIGS. 1-3 are diagrammatic, partial illustrations of a main helicopterrotor of the four-blade kind. It comprises a one-piece hub 1 constructedas follows: its central part 1a is symmetrical in revolution around therotor axis A. The central hub part 1a, together with the top part 2 ofthe tubular strut of the rotor, to which it is connected by afrusto-conical part 1b, forms a single metal component, which is e.g.machined from a forged blank, inter alia of steel or titanium to reduceweight. The peripheral part 1c of hub 1 is substantially circular (asshown at 1f in FIG. 1) and is likewise in one piece with its centralpart 1a and with the rotor strut 2; the hub has a peripheral part 1c inthe form of a flat ring which, in the direction of the rotor shaft A, isformed with a number of apertures 1d equal to the number of rotorblades, i.e. four in the present case. Each aperture, which in plan viewcan have the shape shown in FIG. 1, has an outer edge 1e in a planeparallel to axis A and extends in the radial direction of hub 1 from thecentral part 1a up to a short distance d from the substantially circularperiphery 1f of part 1c. Owing to the aforementioned structure, theassembly formed by strut 2, central part 1a and peripheral part 1c hashigh mechanical resistance, more particularly to centrifugal tension andthe bending stresses applied to part 1c, whereas its weight is much lessthan that of the prior-art hubs, which are completely rigid and usuallyof metal.

A known laminated spherical abutment (general reference 3) is mounted onthe outer edge 1e of each aperture such as 1d through the peripheralpart 1c. In the embodiment under consideration, the laminated abutment,which has a geometrical centre C, comprises a metal core in the form ofa convex spherical cap 3a, made e.g. of titanium or aluminium alloy andcomprising two lugs engaging one above and one below the peripheralregion 1c between aperture 1d and the circular edge 1f (FIG. 1). Thelugs of core 3a are connected to part 1c by a bolt 4 and a nut (FIG. 3).Abutment 3 also has a rigid metal holder 3c made of the same metal ascore 3a and having an internal surface in the form of a concavespherical cap. FIG. 3 clearly shows the shape of holder 3c incross-section through a radial plane of hub 1. A stack 3b of rigid metalcaps in the form of portions of concentric spheres alternating withlayers of elastomer is disposed between the convex spherical surface ofcore 3a and the concave spherical surface of holder 3c. The assemblycomprising core 3a, holder 3c and stack 3b can be secured together byvulcanization to form a body which transmits axial pressure but isresiliently deformable by shearing the elastomer layers in stack 3b sothat core 3a can rotate relative to holder 3c. A conical axial recess 3eis formed in stack 3b to correspond with cylindrical holes 3f and 3gformed in core 3a and holder 3c. Recess 3e and holes 3f, 3g are forinserting the elastomer between the metal components by casting it underpressure before the assembly is vulcanized.

Each of the four blades, e.g. 5, of the rotor illustrated in FIGS. 1-3can have an appropriate internal construction, e.g. can be made ofsynthetic or mineral fibres coated and secured together by a syntheticthermosetting resin, the fibres being embedded in layers of clothimpregnated with a synthetic resin by a known method. The invention isnot limited to the last-mentioned embodiment of the blades, but itspecifies that the holder 3c of each spherical abutment such as 3 ismounted between the ends of the arms 5a, 5b of a forked componentsecured to the root of the corresponding blade 5. More specifically, inthe embodiment illustrated in FIGS. 1-3, the forked component is aprolongation 5a-5b of the root of blade 5, the ends of arms 5a, 5b beingrespectively secured to the top and bottom end of holder 3c, inter aliaby two bolts 6a, 6b each extending through rings engaging in alignedapertures in the end of the two arms 5a, 5b of the root of blade 5 andin an aperture in holder 3c, the bolts being secured by nuts such as 6c(FIG. 3).

According to the invention, a resilient trailing return brace 7 (FIG. 1)is associated with each blade 5. As shown in FIG. 2, each brace 7comprises a stack of metal plates 7a-7c alternating with plates 7d, 7eof a viscoelastic material having great persistence of derformation.Plates 7a-7c are secured together by vulcanization or sticking. One endof each brace 7 is directly coupled to the root of the correspondingblade 5 by an iron mounting 8 and a ball joint 9 mounted at the end of aprolongation of the internal metal plate 7b. The other end of each brace7 is embodied in prolongations of the two outer metal plates 7a, 7c andsecured by bolts 10a, 10b to a short yoke 11, which is coupled by a balljoint 12 to a component 13 secured by a bolt 14 to a place on theperiphery of part 1c between the spherical abutment 3 associated withthe blade 5 in question and the corresponding spherical abutment (notshown in FIG. 1) associated with the immediately preceding blade in thedirection of rotation (indicated by arrow f in FIG. 1). In theembodiment under consideration, bolt 14 secures the internal end ofbrace 7 to the periphery of hub 1 at a place exactly on the bisector Bof two radial axes R₁, R₂ perpendicular to one another and respectivelydefining the neutral position of blade 5 and the neutral position of theimmediately preceding blade in direction f, i.e. the respectivepositions of the longitudinal axes of these blades when the rotor stops(drag angle δ=0). However, it is not necessary for the internal end ofeach brace 7 to be secured in such a precise manner. In theaforementioned preferred embodiment, according to an advantageousfeature of the invention, the centre of the ball joint 12 coupling eachbrace 7 to hub 1 is near the beat axis D of the corresponding blade 5,the axis extending through the axis C of the corresponding sphericalabutment 3 as shown in FIG. 1. In the aforementioned case of a mainrotor, the beat axis D is of course substantially horizontal.

A pitch control lever 15 is secured to the forked component 5a, 5bassociated with the blade 5 on the side remote from the brace 7associated with the same blade 5. In the embodiment illustrated in FIGS.1-3, the pitch control lever 15 has two apertures 15a (FIG. 3) by meansof which it is placed on the threaded ends of the aforementioned bolts6a, 6b so as to be secured by nuts 6c against the end of the bottom arm5b of the root of blade 5. According to another feature of theinvention, the control end 15b of the lever for controlling the pitch ofeach blade is near the beat axis D of the corresponding blade 5 in thesame manner as the aforementioned ball joint 12 but on the other sidefrom joint 12 relative to the centre C of abutment 3.

Finally, the bottom arm 5b of the forked root of each blade 5, at itsend near the rotor, has an abutment 16 disposed so as to limit thedownward beat of blade 5 by co-operating with a reversible ring 17 (FIG.3) mounted in known manner around the rotor strut below hub 1. In theembodiment shown in FIGS. 1-3, the metal abutment 16 forms aright-angle, one arm having apertures for the bottom ends of bolts 6aand 6b so that abutment 16 can be secured by nuts 6c against the end ofthe bottom arm 5b of the corresponding end of the pitch control lever15. Ring 17 is likewise of metal and is mounted so that it can slideradially, with gentle friction, in a radial slot 18 in a component 19secured to the periphery of strut 2. Component 19 can either be single,bounding an annular slot 18 having the appropriate radial depth, oralternatively a number of components 19 can be regularly spaced on theperiphery of strut 2.

FIGS. 1-3 do not show the mechanism for simultaneously or separatelycontrolling variations in the pitch of the various blades 5. Theinvention is not limited to a particular embodiment of such a mechanism,since several embodiments are known. It is sufficient to mention thatthe aforementioned mechanisms, via links, subjects the ends 15b of pitchcontrol levers 15 to substantially vertical upward or downward forces,depending on the desired direction of the change in pitch.

When the rotor has stopped, simultaneous downward pivoting of the fourblades is limited by their respective abutments 16 co-operating withring 17, which is mounted so that it can slide radially. In theinoperative position, the longitudinal axis of each blade 5 is in thesame vertical plane as the radial axis, e.g. R₁, the blade in questionbeing in its neutral position (δ=0).

When the rotor is driven in rotation via strut 2, the system ofcentrifugal forces applied to the various components of each blade 5subjects the corresponding spherical abutment to a radial resultantalong axis R₁, which is absorbed by the compression of the resilientbody 3b of abutment 3. As a result of the various moments andaerodynamic and inertial forces acting on each blade, the blade assumesa beat equilibrium position owing to shearing deformation of body 3caround centre C. The driving torque is transmitted to each blade bycompressing the associated brace 7 so that each blade 5 then occupies a"retarded" position in which its longitudinal axis L co-operates withthe axis R₁ defining the neutral position of the blade to bound a dragangle δ which is substantially the same for all the blades.

Under normal rotor conditions, when the helicopter is flying in astraight line, any oscillations in the drag of each blade 5 will resultin slight variations in the angle formed by axes L and Rl around thepreviously-defined value of angle δ. They are largely absorbed by theaction of plates 7d, 7e (FIG. 2), which are made of visco-elasticmaterial having great persistence of deformation, the two plates formingthe corresponding brace 7. Since ball joint 12, mounted at the internalend of brace 7, is near the beat axis D of blade 5, the beating motionof the blade around its axis D results only in negligible compressive ortensile stresses on brace 7, which therefore has only a negligibleresilient return and shock-absorbing effect on the beat of blade 5. Thesame applies, for the same reason, to the substantially verticalmovement of the end 15b of the pitch control lever 15 associated witheach blade 5.

When the rotor is at rest, the brace 7 associated with each blade 5returns to its original length and shape by subjecting the root of blade5 to a return force which brings it back to its previously-definedneutral position (δ=0), the downward sag of the blade being againlimited by ring 17.

FIGS. 4 and 5 show a four-blade rotor according to the invention whichis likewise a main helicopter rotor but is constructed so that theblades can be folded when the helicopter is parked.

In FIGS. 4 and 5, references corresponding to those in FIGS. 1-3 areused for corresponding components which will not have to be described indetail. We shall therefore only describe those features of theembodiment in FIGS. 4 and 5 which differ from the previously-describedembodiment illustrated in FIGS. 1-3.

As in the previously-described embodiment, each spherical abutment 3 ismounted between (a) the outer edge 1e of the aperture 1d in thedirection of rotor axis A through the peripheral part 1c of hub 1 and(b) the ends of the arms 20a, 20b of a forked component secured to theroot of the corresponding blade 5. In FIGS. 4 and 5, however, the forkedcomponent 20a, 20b mainly comprises two rigid plates 20a, 20bsubstantially parallel to one another and to the peripheral part 1c ofthe rotor, which is in the form of a flat ring. As shown in FIG. 5,plates 20a, 20b are disposed on either side of the peripheral part 1c ofhub 1 without contact therewith, and are also secured to holder 3c ofabutment 3 by two bolts 6a, 6b. The other ends of the two parallelplates 20a, 20b are secured to the root of the corresponding blade 5 bytwo shafts 21a, 21b, at least one of which, e.g. 21a, is movable. Theshaft is e.g. a tubular shaft engaging with gentle friction in matchingapertures in the outer ends of the two parallel plates 20a, 20b and theroot of blade 5. Shaft 21a is normally secured in the aforementionedapertures, e.g. by a metal wire clip 22 which can be removed in order totake out the shaft 21a, after which blade 5 can be folded by pivotingaround the fixed shaft 21a in the direction of arrow F (FIG. 4) towardsthe rear of the helicopter into the parking position. Of course, thiscan be done only after uncoupling strut 7 of blade 5 at the ironmounting 8.

The aforementioned embodiments of a main helicoptor rotor can bemodified in various ways, all within the framework of the invention. Therotor hub according to the invention, instead of forming a single metalcomponent with strut 2, can have a composite structure which, in knownmanner, mainly comprises a stack of cloth layers coated and securedtogether by a hardened synthetic resin, the cloth being made up ofsynthetic or mineral fibres having high mechanical strength. Use can bemade e.g. of glass fibre or synthetic fibre cloth commercially known asKevlar, or carbon fibre cloth, which is even stronger and considerablyless dense, which is advantageous in further reducing the weight of therotor hub according to the invention. Although composite structures ofthe aforementioned kind have hitherto been used to make semi-rigid orpartly flexible rotor hubs, it is also possible to construct similarcomposite structures which are very rigid in order to form the rotorhubs according to the invention. Of course, a hub having a compositestructure of the aforementioned kind must be secured by any appropriatemeans, e.g. bolts, to the top end of the rotor strut, which is usually ahollow metal shaft. Similarly, in the case of a foldable-blade rotor ofthe kind illustrated in FIGS. 4 and 5, the two plates 20a, 20b caneither be of metal or have the previously-described composite structureof hub 1. The two plates 20a, 20b for coupling the root of each blade 5to the corresponding spherical abutment 3 can likewise be replaced by aforked component having a different shape, e.g. a radially disposedyoke, whose end remote from the hub is secured to the correspondingblade root by two shafts substantially perpendicular to the plane of therotor, one shaft being preferably movable so that the blade can befolded by pivoting around the other shaft, whereas the other end of theyoke comprises two flat rigid components disposed one on each side ofthe peripheral part of the hub without contact therewith, and secured tothe holder of the corresponding spherical abutment, e.g. by two bolts.As previously stated, the peripheral part 1c of hub 1 according to theinvention is a flat ring; its periphery 1f can be substantially circularas in FIG. 1, but with gaps on either side of the places 14 where braces7 are secured, or can be a preferably regular convex polygon as in FIG.4. Thus, the rotor hub according to the invention is very markedlydifferent from the star-shaped hubs, i.e. having a concave polygonalperiphery, mentioned in the description of the prior art.

FIGS. 6 and 7 illustrate a rear helicopter rotor according to theinvention which does not differ from the main rotor illustrated in FIGS.1-3 and previously described except in the following particulars: Core3a of each abutment 3 is secured to the peripheral part 1c by a singleradial bolt 4. A tube component 23 having an outer diameter less thanthe internal diameter of strut 2 fits in the hollow central part 1a ofhub 1, to which it is secured by bolts 24. A tube 25 which is longerthan component 23 but has a smaller diameter is mounted so that it canslide freely in component 23, e.g. along longitudinal grooves. Anon-rotary pitch control shaft 26 is mounted along the axis of tube 25.Their (?) front ends, disposed in front of blades 5, are coupledtogether by an abutment ball bearing 27. A star-shaped component 29,technically known as a "spider", is secured by bolts 28 to the peripheryof the front end of tube 25. Component 29 has a number of arms 29a equalto the number of blades 5, and the end of each arm 29a is coupled to theforked component 5a, 5b prolonging the root of blade 5 by a transmissioncomprising a spherical ball joint 30, a link 31 and an iron mounting 32secured to the forked component 5a-5b by the same bolts 33 which, on theother side of blade 5, secure the mounting 8 for holding the brace 7associated with the same blade 5 (see FIG. 6). Bolts 33 extend through awedge-shaped component 34 made e.g. of thermosetting synthetic resinfilled with glass fibres and having triangular surfaces against whichthe mountings 8 and 32 bear.

According to the invention, the two arms 5a, 5b of each forked componentsuch as the foot of blade 5, illustrated inter alia in FIG. 7, haveabutments 16a, 16b at their ends near strut 2, the abutments beingdisposed so as to limit the beat of the corresponding blade 5 on eitherside of the substantially vertical plane of the rear rotor, byco-operating with respective abutments, i.e. an abutment 17a secured tothe tubular element 23 prolonging the strut 2 in the direction of thepitch control device 29, and an abutment 17b secured to strut 2 andprojecting from the outer wall thereof in the form of either one annularprojection or a number of radial projections disposed opposite the rootsof the respective blades.

As in the case of the aforementioned main rotors, each sphericalabutment 3 of the rear rotor allows trailing movements of limitedamplitude by the corresponding blade 5, a shock-absorption effect and aresilient return force being exerted by the corresponding brace 7.Abutment 3 also allows a beat of limited amplitude on either side of thesubstantially vertical plane of the rear rotor, by abutments 16a, 16bco-operating with abutment 17a, 17b respectively. The variations in thepitch of the rear rotor are servo-controlled by axially sliding shaft 26in one or the other direction, along the common axis of tubular elements2, 23 and 25, so as to move the spider 29 as indicated by the doublearrow G, whereas component 29, via bearing 27, can be driven in rotationby rotor blades 5 and components 32, 31, 30. As can be seen, thelast-mentioned components convert each sliding motion of spider 29 inone of the directions of the double arrow G into pivoting of thecorresponding blade 5 around its longitudinal axis L. The pivoting isallowed, with limited amplitude, by the deformation of body 3b aroundthe radial axis through its centre C, body 3b exerting only a weakreturn torque on the blade and also absorbing the resultant of thecentrifugal forces applied thereto.

The rear helicopter rotor according to the invention has the followingadvantages. The dynamic beat and drag torques applied to the variousblades 5 are greatly reduced, thus reducing the alternating stresseswhich they undergo and considerably lengthening their life; if a bladein the rear rotor is damaged, it can be individually replaced, unlikethe case of prior-art rear rotors of the kind comprising a longitudinalmember extending from one end of a blade to the end of the exactlyopposite blade. The brace 7 associated with each blade 5 can be securedto the root of the corresponding blade by the same components, interalia the same bolts 33, as the mounting 32 by means of which the pitchcontrol device 29, 30, 31 actuates the blade root. As shown in FIG. 6,the place 14 where each brace 7 is secured to edge 1c of hub 1 and thecontrol end of mounting 32 are near the beat axis D of the correspondingblade 5, which extends through the centre C of its spherical abutment 3.Furthermore, in contrast to the prior-art rear rotors, wherein each pairof blades is held by a single longitudinal member, the rear rotor hubaccording to the invention has a hollow central part 1a and is easy tomount and the dimensions and weight of the pitch conrol device 25, 26,29 can be reduced by taking advantage of the wide cylindrical ductthrough the tubular strut 2 and the tubular element 23 in the centralhollow part 1a of hub 1. The last-mentioned feature also enables hub 1and spider 29 to be streamlined in a single operation, so as to reducethe drag of the rear rotor according to the invention. The rear rotoraccording to the invention also has reduced mass and is cheaper tomanufacture, and maintenance is reduced to the replacement, wherenecessary, of the laminated spherical abutments and resilient returntrailing braces after a long service life (on average greater than 2,000hours). Finally, the force required for varying the pitch of each rotorblade according to the invention is relatively small, owing to the weakreturn torque produced by the laminated spherical abutment. Consequentlythe invention can obviate the use of a double servo-control device and asingle servo-control device can, in economically advantageous manner, beused for the rear rotor. If the single servo-control fails, the controlforces required for continued flight are so weak that the pilot, withoutexcessive effort, can actuate the rudder bar controlling the pitch ofthe rear rotor.

Some of the previously-described embodiments of main rotors according tothe invention are also applicable to rear rotors according to theinvention.

FIGS. 8-10 are partial diagrammatic views of a main helicopter rotor ofthe four-blade kind. It comprises a rigid hub 1 constructed as follows:its central part 1a comprises a tubular stem having a diameter near thatof the rotor strut 2. The central stem 1a is secured to a top plate 1gwhereas the top part 2a of the rotor strut is secured to a bottom plate1h having an internal edge to which an annular flange 1i, formed on theoutside of the bottom part of stem 1a, is secured e.g. by a ring ofbolts 40. As shown in FIG. 9 plate 1h has the shape substantially of astar having four arms and the same applies to the top plate 1g, theassembly being disposed so that the respective arms of plates 1g and 1hhave the same dimensions and are exactly superposed. As shown in FIG. 8,each plate 1g and 1h is thin, i.e. has nearly the same thickness as thewall of the tubular stem 1a, and the ends of the arms have thickportions 1gl or 1hl formed with two pairs of substantially superposedholes 41. The arms of plates 1g, 1h are e.g. dimensioned so that theaxes of the pairs of holes 41 are substantially parallel to axis A ofhub 1 and of rotor strut 2, and are at a distance from axis A which ise.g. between 3 and 4 times the radius of stem 1a. As shown in FIG. 8,the bottom surface of the bottom plate 1h is reinforced by radial ribs1j. The assemblies formed by (a) the rotor strut 2 and the bottom plate1h and (b) the tubular shank 1a and the top plate 1q, can either eachform a single metal component, e.g. moulded, or can have a compositestructure, mainly comprising a stack of layers of cloth coated andsecured together by a hardened synthetic resin, the cloth being made upof synthetic or mineral fibres having high mechanical strength as statedpreviously.

A known laminated spherical abutment (general reference 3) is mountedbetween the ends of each pair of superposed arms of plates 1q and 1h. Inthe embodiment under consideration, abutment 3 (the geometrical centreof which is denoted by C) has a metal core 3a in the form of a convexspherical cap, made e.g. of titanium or aluminum alloy and integral witha holder comprising a top part 3a1 and a bottom part 3a2 for fittingbetween the ends of the corresponding arms of plates 1q and 1h. At thesides, the holder has two apertures which can be made to coinciderespectively with the two pairs of holes 41 in the ends of thesuperposed arms of plates 1q and 1h, where the bolts 4 can be engaged,thus directly securing abutment 3 via the rigid element 3a to the edgesof the two plates 1q, and 1h of hub 1. Abutment 3 has another rigidelement 3c, made e.g. of the same metal as core 3a and the associatedholder, the internal surface of component 3c being in the form of aconcave spherical cap. A stack 3b of rigid metal caps in the form ofportions of concentric spheres alternating with layers of elastomer isdisposed between the convex spherical surface of core 3a and the concavespherical surface of element 3c. The assembly comprising core 3a,component 3c and stack 3b can be joined together by vulcanization oranother process of sticking, as previously stated.

According to the invention, the root of each of the four blades 5 of therotor illustrated in FIGS. 8 and 9 is secured to the rigid element 3c ofthe corresponding abutment 3 by a radially disposed yoke which isrecessed to provide a free passage for abutment 3. In the embodiment inquestion, the yoke (an exploded view of which is given in FIG. 10)comprises two rigid plates 42a, 42b recessed at 43a, 43b respectively toprovide a free passage for abutment 3, and a cross-member 44 securedbetween plates 42a, 42b e.g. by three bolts 45 extending throughmatching apertures 46a in plate 42a, 46 in cross-member 44 and 46b inplate 42b. As shown in FIGS. 8-10, cross-member 44 is thus secured inthe space between recesses 43a, 43b in two plates 42a, 42b on the onehand and the ends of the plates remote from hub 1, which are also formedwith pairs of matching apertures 47a or 47b. As shown inter alia in FIG.8, the root of the corresponding blade 5 engages with slight clearancebetween the aforementioned ends remote from hub 1 of the two plates 42a,42b. The three components (42a, 5 and 47b) are secured together by twoshafts 21A and 21B; the shafts extend in directions substantiallyperpendicular to the rotor plane through the two pairs of apertures 47a,47b in the two rigid plates 42a, 42b, provided with rings such as 48a,48b, and through the corresponding apertures in the root of blade 5. Atleast one of the two shafts, e.g. 21A, is preferably removable so as tofold blade 5 in the rotor plane by pivoting around the other or fixedshaft 21b as previously described. As shown inter alia in FIG. 8, theends 42a1 and 42b1 nearer hub 1 of plates 42a, 42b are partly fittedinto corresponding gaps in the rigid component 3c of abutments 3, towhich ends 42al and 42bl are secured by a plate 49 which in turn issecured to component 3c by screws 49a in the space between theaforementioned gaps.

In the last-mentioned embodiment, the cross-member 44 secured betweenplates 42a, 42b of the yoke is prolonged beyond the yoke, on the sameside as the leading edge of blade 5 (i.e. in the direction of rotationof the rotor indicated by arrow f in FIG. 10) by a bent continuation44a; as shown inter alia in FIGS. 9 and 10, continuation 44a starts in arecess 44b open on the side of hub 1, having two side walls formed withapertures 44b1 and 44b2 aligned along an axis D extending through thecentre C of abutment 3, which is preferably in the same plane as theaxes of the screws 4 securing abutment 3 to the hub plates 1g and 1h.Beyond recess 44b, the continuation 44a of cross-member 44 bears a yoke44c having two lugs formed with holes 44c1 and 44c2 aligned along anaxis E substantially perpendicular to axis D.

According to the invention a resilient trailing return brace 7,preferably constructed as previously described, is associated with eachrotor blade 5 as follows: The first end of brace 7 (i.e. furthest fromhub 1) is coupled by a ball joint 9 to a shaft 49 extending through therecess 44b of the bent continuation 44a and the apertures 44b1 and 44b2in its walls, to which the ends of shaft 49 are connected by anyappropriate means. The second end of brace 7 (i.e. nearer hub 1) iscoupled to the tubular stem 1a of hub 1 by a ball joint 12 secured by abolt 50 in an annular region 51 of stem 1a, where the wall of stem 1a isinternally thickened as shown in FIG. 8. In the embodiment shown, theball joint 12 coupling the second end of brace 7 associated with a blade5 is secured to stem 1a at the yoke associated with the immediatelypreceding blade in the direction of rotation of the rotor (f in FIG.10), in the space between stem 1a and the yoke.

A shaft 52 engages in the two holes 44c1 and 44c2 of yoke 44c ofcross-member 44 and is secured by any appropriate means and acts as apivot for the end 15b of the pitch control lever of the correspondingblade 5. Like ball joint 9, end 15b is coupled to the bent continuation44a near the axis D extending through the centre C of the sphericalabutment 3.

As shown at the bottom of FIG. 8, the edge of the bottom plate 1h of hub1 bears an abutment 54 at each blade 5, i.e. at the end of thecorresponding arm of plate 1h. Abutment 54 is of a known kind and can beretracted in flight, e.g. by centrifugal force. The bottom surface ofthe rigid plate 42a also has a projection 56 which can co-operate withthe retractable abutment 54 when the abutment is in the operatingposition and the rotor is stationary or rotating slowly.

When the rotor is stationary the four blades pivot simultaneouslydownwards because of their weight, but the motion is limited byabutments 56 of the respective yokes, inter alia on the bottom plate42b, which co-operate with the corresponding abutment 54, which is thenin the operating position.

When the rotor is driven in rotation via strut 2, the system ofcentrifugal forces applied to the various components of each blade 5subjects the corresponding spherical abutment 3 to a radial resultantwhich is absorbed by compressing the resilient body 3b of abutment 3. Inaddition, as a result of the various aerodynamic forces and moments andthe inertia on each blade, the blade takes up a beat equilibriumposition as a result of shearing deformation of the resilient body 3caround centre C of abutment 3. The driving torque is transmitted to eachblade by the associated brace 7, which is thus slightly stretched, sothat the position of blade 5 is slightly behind its neutral position,when the rotor is at rest.

When the rotor is operating normally and the helicopter is flyingstraight, the drag oscillations of each blade 5 are mostly damped by theresilient return brace 7 as previously described. Ball joint 9, at whichthe first end of brace 7 is coupled to the root of the correspondingblade 5 via cross-member 44 and its continuation 44a, is near axis Dextending through the centre C of abutment 3, which constitutes the beataxis of blade 5. Consequently the beat motion of blade 5 around its axisD produce negligible compressive or tensile stresses on brace 7, andconsequently brace 7 has a negligible resilient return and dampingeffect on the beat of blade 5. The same applies to the substantiallyvertical motion of end 15b of the pitch control lever associated withblade 5, since end 15b is pivoted on shaft 52 near the beat axis D ofblade 5. In addition, since abutments 54 are then retracted as a resultof centrifugal force, the downward beat of blades 5 is not limitedduring normal operation.

When the rotor stops, the brace 7 associated with each blade 5 returnsto its normal length and shape by subjecting the root of blade 5 to areturn force which brings it into the neutral position, whereupon thedownward sag of the blades is again limited by abutments 54, whichreturn to the operating position when the rotor speed falls below agiven value.

Part of a variant of the rotor hub 1 illustrated in FIGS. 8-10 is shownin FIG. 11, in which like references have been used to denote likecomponents. In FIG. 11, the top plate 1g of hub 1 is as before securedto its central stem 1a, but the bottom plate 1h is an independentcomponent from the top part of the rotor strut 2. The three components1a, 1h and 2 are assembled by means of their respective annular flanges1i, 1k and 2b, using a ring of bolts 40.

FIGS. 12 and 13 diagrammatically show another embodiment of theinvention, likewise using the same references for like components as inFIGS. 8-10. The last-mentioned embodiment is identical with thepreviously-described embodiment except in the following points.Cross-member 44, which is secured between the two plates 42a, 42b of theyoke associated with each blade 5, is prolonged by a first bentcontinuation 44a1 beyond the yoke on the leading-edge side of balde 5,i.e. in the direction of arrow f indicating the direction of rotation inFIG. 12, whereas member 44 is prolonged by a second bent continuation44a2 on the trailing-edge side. Continuation 44a1 has only two lugs 44c,perforated at 44c1 and 44c2 respectively for securing the pivot 52 ofthe end 15b of the pitch control lever of the corresponding blade. Thesecond bent continuation 44a2 is formed with a recess 44b extending allthe way through shaft 49 securing the ball joint 9 associated with thefirst end of brace 7 corresponding to blade 5. In the last-mentionedembodiment, therefore, each resilient trailing return brace 7 is on thetrailing-edge side of the corresponding blade, and is thus slightlycompressed during normal rotor operation. As in the precedingembodiment, end 15b and the pivot 9 of the first end of the associatedreturn brace 7 are near the beat axis D of blade 5, with thepreviously-mentioned results.

The embodiment of the retractable abutments 54 is optional, and the sameapplies to the manner of constructing the cross-members 44 illustratedin perspective in FIGS. 10 and 13. Instead of comprising a singlecross-member simultaneously bearing the pivot of the end of the pitchcontrol lever and the ball joint of the first end of the resilientreturn brace, the yoke associated with each rotor blade according to theinvention could comprise two independent cross-members for separatelyperforming the two functions of the single previously-describedcross-member. Instead of being formed by two rigid plates 42a, 42bconnected by at least one cross-member 44, the yoke for connecting theroot of each blade 5 to the corresponding abutment 3 could bedifferently constructed, e.g. in the form of a single moulded ormachined component formed with a suitable recess providing a freepassage for abutment 3. The yoke may likewise be secured to and integralwith the root of the corresponding blade 5. This last embodiment issimpler mechanically but of course would not allow the blades to fold.The method of securing the ends nearer the hub of the yoke plates 42a,40b is optional, and the same applies to the construction of thespherical abutment 3. The reinforcing ribs 1j of the bottom plate 1h canbe omitted if the bottom plate is sufficiently thick. Alternatively thehub assembly, inter alia the central stem 1a and plates 1g and 1h canco-operate with the top part of the rotor strut 2 to form a singlecomponent, made e.g. by moulding.

What I claim is:
 1. A rotary aircraft rotor comprising a hub arrangedfor rotation about a rotor axis and supporting a plurality of rotorblades presenting each a root,said hub being formed in the direction ofthe number of blades, and being coupled to the root of each blade bymeans of a laminated spherical abutment mounted between an outer edge ofthe corresponding aperture and inner ends of two arms of a forkedcomponent intergral with the corresponding blade root, and having acenter through which extends the blade beat flap axis, and of aresilient trailing return brace comprising an elongated stack of metalplates alternating with elastomeric shock-absorbing plates made of viscoelastic material having great persistence of deformation, a firstextremity of said brace being coupled to the corresponding blade root,wherein said rotor hub is a rigid one-piece hub having a peripheral partin form of a flat ring, the periphery of which is of the type comprisingconvex polygonal and substantially circular peripheries, said aperturesbeing formed in said hub peripheral part, and each of said braces havingtwo extemities each equipped with a ball-joint via which said firstextremity is coupled to the corresponding blade root and the secondextremity is coupled to a place on the hub periphery between thelaminated sperical abutment of the corresponding blade and the laminatedsperical abutment of an immediately neighbour blade said ball joint ofsaid second brace extremity having its center near the beat flap axis ofthe corresponding blade.
 2. A rotor as in claim 1, wherein a pitchcontrol lever is secured to each forked component on the side remotefrom the brace of the corresponding blade, and has a free end near thebeat flap axis of said corresponding blade.
 3. A helicopter rear rotoras in claim 2, wherein said two arms of each formed component haveabutments disposed at their inner ends and limiting the beat of thecorresponding blade on either side of the rotor rotation plane byco-operating with respective abutments, one of which is secured to arotor strut fixed to a hollow central part of said rigid hub and theother secured to a tube component partly extending in said strut andsaid hub hollow central part and partly extending in the prolongationthereof.
 4. A helicopter rear rotor as in claim 3, wherein a non-rotarypitch control shaft extends coaxially in a slide tube coaxially mountedin said tube component, said pitch control shaft and said slide tubehaving end parts in the prolongation of said tube component and coupledtogether through an abutment ball bearing, the end part of said slidetube carrying a star shaped component having for each blade an armextending in a plane perpendicular to said slide tube, said armpresenting a free end coupled to the free end of the corresponding pitchcontrol lever through a link.
 5. A rotor as in claim 2, wherein saidforked components are each a prolongation of the corresponding bladeroot, and said ends of its arms are secured by two bolts to a rigidholder presenting a concave spherical cap of the corresponding laminatedspherical abutment.
 6. A rotor as in claim 2, wherein said forkedcomponents each mainly comprises a radially disposed yoke the outer endof which is secured to the corresponding blade root whereas the innerend of which comprises two rigid flat elements secured to one another,disposed on either side of said peripheral part of the hub withoutcontact therewith, and secured to a rigid holder presenting a concavespherical cap of the corresponding laminated spherical abutment.
 7. Arotor as in claim 6, wherein each yoke is made of two rigid platessubstantially parallel to one another and to said peripheral part of thehub, and secured to the corresponding blade root by two shaftssubstantially perpendicular to said plates.
 8. A rotor as in claim 7,wherein one of said shafts is removable to allow the folding of thecorresponding blade in the rotor rotation plane by pivoting around theother shaft.
 9. A rotor as in claim 2, wherein said inner end of thebottom arm of each said forked component has an abutment limiting thedownward beat of the corresponding blade when cooperating with areversible ring slidably mounted in radial direction around a rotorstrut on the upper part of which said rotor hub is fixed.